For optical transmission systems, erbium-doped fiber amplifiers (EDFAs), which are capable of collectively amplifying a wide wavelength region, are widely spread. However, in an optical transmission system that uses an EDFA as an optical amplifier, ASE (Amplified Spontaneous Emission) generated by the EDFA degrades an optical signal to noise ratio (OSNR). Accordingly, Raman amplifiers have tended to be widely spread as one type of an optical amplifier with little degradation in the OSNR.
In comparison with EDFAs, Raman amplifiers have low pump-light conversion efficiency. That is, to obtain sufficient gain using a Raman amplifier, a high-power pump light is supplied to a transmission optical fiber. Thus, a Raman amplifier may need to have a function to instantly stop pump light when a transmission optical fiber is disconnected.
Meanwhile, before an optical transmission system starts to be operated, a Raman amplifier measures the relationship between pump-light power and the power of ASS (Amplified spontaneous Raman Scattering). The relationship between pump-light power and ASS power does not depend on whether or not an optical signal is present. Thus, measuring in advance the relationship between pump-light power and ASS power allows the Raman amplifier to calculate the ASS power according to the pump-light power even while the optical transmission system is being operated. Accordingly, the Raman amplifier may detect the power of a pure optical signal by deducting the ASS power from the optical power detected by an optical receiver, thereby precisely controlling the gain.
Related arts are described in, for example, Japanese Laid-open Patent Publication No. 2004-287307, Japanese Laid-open Patent Publication No. 2002-252595, and Japanese Laid-open Patent Publication No. 2009-159290.
In the process of determining the aforementioned relationship between pump-light power and ASS power, the ASS power is measured while gradually changing the pump-light power. However, in Raman amplification it takes a long time to stabilize the gain after pump power is changed. In particular, a Raman amplifier provided with a cheap laser and/or a simplified driving circuit to decrease the cost may take about one second to stabilize the gain after pump-light power has been changed. Thus, when a transmission optical fiber is disconnected during the process of determining the relationship between pump-light power and ASS power, the Raman amplifier may be unable to instantly detect the fiber disconnection.
As described above, conventional Raman amplifiers may be unable to determine the state of a transmission optical fiber for a long time. Thus, the prior art may be incapable of instantly stopping pump light when a transmission optical fiber is disconnected.